1. Field of the Invention
The present invention relates to a switching power supply device, and more particular to a switching power supply device suitable as a power supply for an electronic video device, such as a multifunctional video device having a television (hereafter referred to as “TV”) receiver with a video cassette recorder built therein (i.e. electronic video device having both TV receiver function and video cassette recorder function in one housing, which will be hereafter referred to as “TV with built-in VCR”), that is required to provide power supply both to a relatively large load, or large current, such as a CRT (Cathode Ray Tube) and a fluorescent tube for video output, and to a relatively small load such as an electronic control unit.
2. Description of the Related Art
Generally, a switching power supply device is used in an electronic device to obtain multiple voltages from one transformer for the electronic device. For example, a switching power supply device to be used as a power supply device for an electronic video device, such as a TV with built-in VCR or another electronic multifunctional video device e.g. having both TV receiver function and DVD (Digital Versatile Disc) player function in one housing, can easily generate various voltages for the electronic video device from a secondary DC power supply circuit of the switching power supply device. The various voltages include a voltage (typically 120V) for a CRT circuit including a flyback transformer, a voltage (typically 5V) for a control unit (control circuit) having a microcomputer mounted thereon, a voltage for a video/audio signal processing circuit, and a voltage for a loading motor for a video cassette or a video disc.
For example, a secondary DC power supply circuit of a switching power supply device has a 5V power supply line to which a control unit including a microcomputer is connected, and also has a 120V power supply line to which a CRT circuit including a flyback transformer is connected. Normally, the CRT circuit is a largest load, so that the 120V power supply line can be referred to as a large load power supply line, while the 5V power line can be referred to as a small load power supply line. The large load power supply line for the CRT circuit is connected to a voltage signal feedback line from the secondary DC power supply circuit to a primary DC power supply circuit of the switching power supply device.
On the other hand, for power saving, an electronic video device such as a TV receiver or a TV with built-in VCR is designed so that only a control unit including a microcomputer operates until a power supply ON command is input, i.e. in power supply standby state or mode, while the power supply is provided to e.g. a CRT circuit to start up the screen of the CRT only when the power supply ON command is input by a user operating a remote control. In a device of this kind, a dummy resistor is generally connected to an appropriate position in a secondary power supply circuit of the switching power supply device in order to reduce load fluctuation in or during the transition period from the power supply standby state to the power supply ON state, and to stabilize the voltages of secondary power supply lines. For the voltage stabilization, the dummy resistor is typically connected to the large load power supply line, as described above, in which the magnitude of the dummy resistor is normally set to be 10 to 30%, in some cases 50%, of a load imposed on the large load power supply line.
Due to the significant magnitude of the dummy resistor, the dummy resistor connected to the secondary power supply circuit, typically the large load power supply line, of the switching power supply device causes certain significant power consumption even in the power supply standby state. This is a waste of power, and is against the power saving. In order to solve this problem, one may consider using known technologies such as a technology disclosed in Japanese Laid-open Patent Publication Hei 9-65654 to disconnect or separate the dummy resistor in the power supply standby state from the circuit such as the large load power supply line, and to connect the dummy resistor to the large load power supply line only when the state of the circuit changes from the power supply standby state to the power supply ON state.
However, in the case of an electronic video device in which, as described above, a secondary DC power supply circuit of a switching power supply device provides a voltage to a large load such as a CRT circuit, and also provides a voltage to other relatively small loads such as a video/audio signal processing circuit, a problem initiated by the power supply ON operation of a user may occur as will be described below if the change from power supply standby state to power supply ON state (normal load state) and the connection of a dummy resistor are done at the same time. In this regard, it is to be noted that the switching power supply device is generally designed so that the voltages in the power supply standby state provided to the respective power supply lines, and hence the respective loads, are much lower than those in normal state (power supply ON state), and that the variation of the voltages of the power supply lines not connected to the voltage signal feedback line is coupled or analogous to the variation of the voltage of the power supply line connected to the voltage signal feedback line (for example, when the latter voltage is in a transient state, the former voltages are also in a transient state).
When a user inputs a power supply ON command signal, using a remote control, to initiate the change from the power supply standby state to the power supply ON state, a control unit having received the input power supply ON command signal connects the dummy resistor to the large load power supply line, and at the same time starts providing power supply to the electronic video device, more specifically to a video/audio signal processing circuit and a loading motor for a video cassette or a video disc (to start the operation of the electronic video device). This may cause a problem of insufficient current or current shortage in portions of the electronic video device such as the loading motor that require a relatively large current, causing bad operation of the electronic video device, because immediately after the dummy resistor is connected, the secondary DC power supply circuit is in a transient state and is provided with only a small current or insufficient current (current being particularly small in a small load power supply line to which a feedback line is not connected). More specifically, a problem may arise such that when a user inputs a command to remove a video cassette immediately after the user operates a power supply ON key of the remote control, the loading motor is not provided with a sufficient amount of rotation, failing to eject the video cassette.
Besides, in regards to disconnection or separation of a dummy resistor from a secondary circuit, Japanese Laid-open Patent Publication Hei 5-236743 discloses a technology to detect the magnitude of a secondary load, and disconnect a dummy resistor if the load exceeds a predetermined magnitude. However, this technology does not solve the above-described problem.